1. Field of Invention
This application relates to color transformation, and more specifically to the use of matrices and look-up tables for color transformation.
2. Description of Related Art
The following patents are specifically incorporated by reference: U.S. Pat. No. 6,643,029 to Kumada et al. for its teaching of an image processing method using look-up tables (LUTs); U.S. Pat. No. 6,360,007 to Robinson et al. for its teaching of an optimized color LUT transformation based upon image requirements; and U.S. Pat. No. 5,412,491 to Bachar for its teaching of a color transformation apparatus using LUTs.
The generation of color documents often includes two steps, an image input step and an image output step. In the image input step, image signals such as red, green and blue signals in an RGB color space, representative of an image, are produced by a scanner or other image input device. The color signals generated by image input devices are usually device-dependent, in other words, the same RGB values might represent different colors for different input devices. In the image output step, the image is printed by a color printer or other image output device. Most color output devices either use cyan, magenta and yellow signals in a CMY color space or in a CMYK with an additional color signal for black. The color signals used for image output devices are also usually device-dependent. Instead of transferring color signals in the RGB space from a scanner directly to color signals in the CMYK space to a color printer, a device-independent color space is commonly used as the intermediate color space for other evolved image processing, such as compression, decompression, enhancement, correction, and the like. The device independent color spaces are also often standard color spaces, such as the XYZ space, with X, Y and Z color signals, the CIE Lab space with L*, a* and b* signals, and the YCbCr space with Y, Cb and Cr signals. The device-independent color spaces unambiguously define color information that is not limited by the dynamic range or color gamut of any physically realizable device. The most common color transformations are those for converting colors between a device-dependent color space and a device-independent color space. For the image input step, the transformation might be from an RGB color space to the CIE Lab color space. For the output step, the transformation might be from the CIE Lab color space to a CMYK color space.
In order to overcome the device-dependency of image input and image output devices, each device is typically provided with a unique color transformation unit for converting image signals both to and from a device-independent color space.
Using a combination of several one-dimensional look-up tables and a M×N matrix is a common method for color transformation. Many devices, such as, for example, Xerox DigiPath™ scanners, are built with special circuits using 3 LUTs and a 3×3 matrix for color conversion with a high speed data rate. In addition, more than one color conversion may be needed. Thus, some devices are equipped with more than one 3 LUT+3×3 Matrix unit. For example, DigiPath scanners have both RGB to CIE Lab and RGB to YCbCr conversion circuits built into their systems.